A lead frame formed by a metal body is used as a component for electrical connection in semiconductor elements such as LEDs, LSIs, etc. The semiconductor element is electrically connected to the lead frame by wire bonding. Along with the lead frame, the semiconductor element is sealed with resin during production and is then connected to a substrate or the like in a semiconductor device using, for example, solder.
Excellent wire bonding properties and solderability are required in the manufacturing process of the semiconductor device. To guarantee these properties, a plating layer of a precious metal is sometimes provided in the electrical connection component such as the lead frame. Caution is required, however, as the plating layer may peel off or may become lacking. In other words, if a plating layer that has been provided peels off at a later point, a conductive base or lower plating layer will undergo chemical changes such as corrosion or oxidation, thus worsening the wire bonding properties and the solderability. Therefore, it is important for the provided plating layer to have excellent adhesive properties so that the plating layer does not peel off.
One known technology for addressing this problem is as follows. As shown in the cross-section diagram in FIG. 6A, a lead frame is formed from a conductive base 300, a nickel lower plating layer 301 thereon, and a silver uppermost plating layer 303. Between the lower plating layer 301 and the uppermost plating layer 303, a copper intermediate plating layer 302 is provided, thus guaranteeing adhesive properties between the lower plating layer 301 and the uppermost plating layer 303. An improvement in adhesive properties is sought by interdiffusion of copper, which forms a solid solution with both the nickel and the silver.
Another known technology is shown in the cross-section diagram in FIG. 6B. As a lead frame for a semiconductor device, a nickel lower plating layer 401, palladium or palladium alloy intermediate plating layer 402, and a gold uppermost plating layer 403 are formed in this order on a conductive base 400. In this structure as well, excellent adhesive properties between plating layers are guaranteed by using an intermediate plating layer 402 between the lower plating layer 401 and the uppermost plating layer 403, thus attempting to achieve excellent wire bonding properties and solderability.
[Citation List]
[Patent Literature]
    [Patent Literature 1] WO 2007/119522    [Patent Literature 2] Japanese Patent No. 3998703[Summary Of Invention][Technical Problem]
When a silver uppermost plating layer is formed on the lead frame, the following problems occur.
As shown in FIG. 7A, in which a lower plating layer 501 and uppermost plating layer 503 are formed in this order on a conductive base 500, oxygen in the atmosphere penetrates into the silver in the uppermost plating layer 503. As a result, the plating in the lower plating layer 501 oxidizes, and an oxide film 501a forms near the boundary surface with the uppermost plating layer 503. Due to the oxide film 501a, the adhesive properties between the uppermost plating layer 503 and the lower plating layer 501 at the boundary surface of these layers become weaker, and these layers may peel apart.
In the structure shown in FIG. 6A, on the other hand, copper in the intermediate plating layer 602 gradually diffuses in the direction of thickness of the layers, as shown in FIG. 7B. If this copper diffuses to the top surface of the uppermost plating layer 603, the copper reacts with oxygen in the atmosphere and oxidizes, forming an oxide film 603a. The oxide film 603a not only reduces wire bonding properties and solderability, but also increases contact resistance.
Although, in order to solve these problems, the intermediate plating layer 602 may be made thinner to control the diffusion of copper, the thickness of the intermediate plating layer 602 then becomes insufficient, and holes form in the intermediate plating layer 602. These holes lead to the problems of corrosion and oxidation of the conductive base 600 and the lower plating layer 601.
As shown in FIG. 6B, gold may also be used as the uppermost plating layer 403. Gold is an extremely stable precious metal with excellent anti-corrosion properties. Using gold as the uppermost plating layer achieves excellent wire bonding properties and solderability. Since gold is an expensive precious metal, however, thick gold plating results in an extremely high cost. Conversely, whereas a thin plating reduces cost, holes may form in the gold plating constituting the uppermost plating layer 403. Oxygen, water, etc. in the atmosphere may penetrate through the holes, leading to corrosion and oxidation of the conductive base 400 and the lower plating layer 401.
Furthermore, a problem with the manufacturing process of such a lead frame is that the lower plating layer, intermediate plating layer, and uppermost plating layer each have to be formed as separate plating layers, which reduces efficiency of manufacturing. An enormous manufacturing cost is thus necessary for facilities and equipment to perform such predetermined plating processes.
The present invention has been conceived in light of the above problems, and it is an object thereof to provide a lead frame for a semiconductor device and a method of manufacturing of the same that improve adhesive properties between plating layers when a plurality of plating layers are laminated, that control, during the manufacturing process of a semiconductor device, deterioration in wire bonding properties and worsening of solderability when packaged, and that effectively reduce manufacturing cost.
[Solution to Problem]
In order to solve the above problems, a lead frame for a semiconductor device according to the present invention comprises: an electrically conductive base; a lower plating layer formed on the electrically conductive base; an organic film formed on the lower plating layer; and an uppermost plating layer formed on the organic film, wherein the organic film has metal-binding properties with respect to the lower plating layer and the uppermost plating layer.
The lower plating layer, or both the lower plating layer and the uppermost plating layer, may be formed of a precious metal.
The organic film may include a plurality of organic molecules each having a molecular configuration with two or more polar groups, and one of the polar groups in each organic molecule may be bound to metal forming the lower plating layer, so that the organic film coats the lower plating layer.
The organic film may be formed of organic molecules that self assemble, and the organic molecules may have, at both ends, a functional group with metal-binding properties.
A main chain of each of the organic molecules may be a compound, chemical structure, or derivative including at least one selected from the group consisting of an aryl skeleton, an acene skeleton, a pyrene skeleton, a phenanthrene skeleton, a fluorene skeleton, and a nitrogen-containing heterocycle containing at least two nitrogen atoms.
The nitrogen-containing heterocycle may be a compound, chemical structure, or derivative including at least one selected from the group consisting of imidazole, triazole, tetrazole, oxadiazole, thiadiazole, pyrimidine, pyridazine, pyrazine, and triazine.
The functional group with metal-binding properties may be a compound, chemical structure, or derivative including at least one selected from the group consisting of a thiol compound, a sulfide compound, and a nitrogen-containing heterocycle compound.
A method of manufacturing a lead frame for a semiconductor device according to the present invention comprises the steps of (a) forming a lower plating layer on a conductive base; (b) forming an organic film on a surface of the lower plating layer; and (c) forming an uppermost plating layer on a surface of the organic film, wherein the organic film formed in step (c) has metal-binding properties with respect to the lower plating layer and the uppermost plating layer (FIG. 3).
Step (b) may include creating a dispersion liquid by dispersing an organic compound in a solvent and dipping the conductive base on which the lower plating layer is formed into the dispersion liquid (FIG. 4).
[Advantageous Effects of Invention]
In the lead frame for a semiconductor device according to the present invention, a predetermined organic film having metal-binding properties is formed between (i) a lower plating layer formed on a conductive base and (ii) an uppermost plating layer. As a result, the lower plating layer and the uppermost plating layer form a metal bond through the organic film, thus guaranteeing strong adhesive properties in the lower plating layer and the uppermost plating layer. Excellent adhesive properties between plating layers are thus achieved without forming an intermediate plating layer as in a conventional structure, thereby simplifying the structure.
Furthermore, since the lower plating layer is coated by the organic film, the organic film prevents diffusion of metal from the lower plating layer. The organic film also resolves the problems of oxygen in the atmosphere penetrating to the lower plating layer, leading to deterioration in wire bonding properties and solderability, and to an increase in contact resistance.
By appropriately setting the degree of density of the organic film and the structure of the organic molecules, the organic film not only blocks oxygen in the atmosphere, but is also highly effective at blocking corrosive gasses, humidity, moisture, etc. Therefore, providing an organic film with predetermined properties controls corrosion and oxidation of the conductive base and the lower plating layer and prevents formation of an oxide film on the plating boundary surface. This improves adhesive properties between plating layers and prevents the layers from peeling apart. Therefore, even if holes form in the uppermost plating layer, problems such as corrosion or oxidation of the conductive base and the lower plating layer are prevented by providing an organic film.
By using such an organic film that prevents corrosion and oxidation of the lower plating layer as well as diffusion of the metal in the base, the uppermost plating layer can be formed thinner than in a conventional structure, which reduces the cost of materials. Furthermore, forming a thinner plating layer reduces the time required for plating, which not only increases manufacturing efficiency, but also contributes to cost reduction by reducing manufacturing lead time.
The lead frame for a semiconductor device according to the present invention does not depend on the facilities and equipment to form an intermediate plating layer as in conventional technology, thus achieving a major reduction in manufacturing cost. The present invention thus provides a lead frame for a semiconductor device that is inexpensive, highly efficient to manufacture, and that has excellent electrical connection properties.
From the perspectives of functionality, structure, etc., the lead frame according to the present invention therefore has completely different characteristics than a conventional lead frame in which general plating, surface treatment, etc. have been applied to conductive base material.